It is required that a polarization controller for converting an arbitrary polarization to a constant linear polarization or another arbitrary polarization is provided in an optical system such as an optical communication system, an optical fiber sensor and so on. Among polarization controllers, a waveguide type of a polarization controller is advantageous in that it is light weight, has a high productivity, and can be integrated with other devices easily so that the polarization controller is expected to be widely put into practical use.
A waveguide type of a conventional polarization controller has been described in a report entitled "Optical waveguide polarization controller in Z-propagation LiNbO3" which was published as "NATIONAL CONFERENCE RECORD, 1986, OPTICAL AND RADIO WAVE ELECTRONICS, THE INSTITUTE OF ELECTRONICS AND COMMUNICATION ENGINEERS OF JAPAN". The conventional polarization controller is of a tandem structure which is composed of a phase shifter and a mode converter, and comprises an optical channel waveguide (simply called "a waveguide" hereinafter) extended along Z-axis on a LiNbO3 substrate, first and second electrodes provided on both sides of the waveguide in a region of the phase shifter of the substrate, and first to third electrodes provided on the waveguide and both sides thereof in a region of the mode converter.
In the phase shifter, the first electrode is connected to ground, and an adjustable voltage V1 is applied to the second electrode. In the mode converter, the first electrode is connected on the ground side of the waveguide, an adjustable voltage V2 is applied on the waveguide to the second electrode, and a constant voltage V3 is applied on the other side of the waveguide to the third electrode. As a result, birefringence is changed in a direction orthogonal to the light propagation direction of the waveguide and parallel to the surface of the substrate in the region of the phase shifter. In the region of the mode converter, on the other hand, the birefringence of the waveguide is compensated to result in an isotropic condition in accordance with the applied voltage V3, and an extent of a birefringence is changed in a direction orthogonal to the light propagation direction of the waveguide and in a direction of 45 degrees in regard to the surface of the substrate. If it is assumed that a phase difference between TE and TM modes is .pi./2 just in front of the mode converter by adjusting the voltage V1 in the phase shifter, an input light of an arbitrary polarization which is incident to the polarization controller is converted to an output light of a linear polarization which is supplied therefrom to a next stage when an amplitude ratio of the TE and TM modes is controlled to be an appropriate ratio by adjusting the voltage V2 in the mode converter.
In addition to the polarization controller described above, another polarization controller has been proposed in U.S. Pat. No. 4,691,984 which was issued on Sept. 8, 1987. The latter polarization controller comprises a further phase shifter which is the same structure as that of the phase shifter in the former polarization controller. In other words, the latter polarization controller comprises a first and second phase shifters and a mode converter which is positioned between the first and second phase shifters wherein the mode converter is also the same structure as that of the mode converter in the former polarization controller.
In operation, the same adjustments of voltages are performed in the first and second phase shifters and the mode converter as in the former polarization controller described above so that an input light of an arbitrary polarization which is incident to the polarization controller is converted to an output light of another arbitrary polarization which is supplied therefrom to a next stage.
According to the conventional polarization controller as formerly described, however, there is a disadvantage that the aforementioned voltage V1 must be kept increasing or decreasing to convert a phase difference of TE and TM modes of an input light, which is incident to the phase shifter, to .pi./2 successively in a case where the phase difference continues to be increased or decreased. In this regard, it is not allowed to increase or decrease the voltage V1 which is applied to the second electrodes to an unlimited extent because the strength of the insulation of the second electrode is limited. This results in the impossibility of controlling an arbitrary polarization in the phase shifter. That is, this means that there is a limitation for the operating range in controlling a fluctuation of a polarization in an input light which is incident to the phase shifter.
Even in the polarization controller as proposed in the aforementioned U.S. Pat. No. 4,691,984, the same disadvantage is observed.